In the oil and gas industry, it is imperative to know and understand the material properties of compositions used during drilling and exploration and to determine how these properties are affected by temperature, pressure, and time, including determination of cement mechanical and static strength variations as a function of time, temperature, and pressure.
Static gel strength is a measurement of shear strength (stress) and is derived from force required to initiate flow of a static fluid. Static gel strength may be measured in pascals (newtons per square meter) or in pounds per area unit.
Conventional gel strength measurement systems use a paddle in a test cell. The cement is placed in the test cell at determined pressure and temperature and allowed to begin curing. The paddle is rotated in relation to the test cell. Torsional stress is determined from forces on the test cell at various time intervals to determine gel strength at user defined time intervals.
Conventional systems for measuring gel strength include a Chandler Model 5265 SGSA apparatus (manufactured by Chandler Engineering, Inc., Broken Arrow, Okla.) for measurement of gel strength and compressive strength of a cement slurry, a Fann Instruments Model 101677665 MACS II Multiple Analysis Cement System (Manufactured by Fann Instrument Company, Houston, Tex.), and a CTE Model 15-400 RP Consistometer/Gel Strength apparatus (Manufactured by Cement Test Equipment, Inc., Tulsa Okla.). The Chandler apparatus measures gel strength using an acoustic transducer. The Fann Instruments system and the Cement Test Equipment apparatus measure gel strength with a torque transducer.
A conventional method of accounting for friction and other forces in the testing assembly is to determine an initial peak value of system force with a known sample, such as water, using such peak value as a base value, and assigning such value as the “zero value” of the system. This method may result in relatively large measurement errors of the sample as the system forces vary as the paddle or sample is rotated. The relative error may be particularly high when testing low viscosity samples. In addition, deduction of a zero value from sample measurements and consequent disregard for values below zero ignores potentially valuable sample measurements.
The paddle of conventional systems comprises two flat elements on opposing sides of a central rotating shaft, each flat element attached at one edge to the shaft. The two elements are aligned to extend in a common plane.